Flowrate control of liquid furnace products leaving metallurgical vessels and transport to the next process location in launders is very important to the overall performance of such vessels.
In the extractive metallurgy industry, tapping of furnaces is a very difficult, labour intensive and hazardous operation. Conventional tapping of furnaces is carried out periodically through a water cooled breast tapping hole. The hole is opened with an oxygen lance and closed by freezing in the tapping hole assisted by a clay plug or water cooled restrictor bars.
The problems associated with conventional tapping systems include delay in tapping due to difficulties in opening the taphole, no control of tapped liquid flowrate, wear and erosion of taphole due to oxy-lancing and difficulties in closing the taphole. As a result improved tapping techniques are required to overcome these difficulties. One such technique is the continuous tapping concept.
The development of continuous tapping began as early as 1899, when an external forebay for continuous tapping of iron blast furnaces was suggested. This idea was developed further to essentially the present day furnace/forebay relationship. Initially continuous tapping was limited to copper blast furnaces. The next development was the Roy type tapper in which the major differences from earlier developments were the use of an adjustable weir height by means of a deep V-notch and the installation of a burner in the forebay. The Roy type tapper is now conventional technology on lead blast furnaces.
The operation of the Roy type tapper is based on the principle of the liquid in the forebay counterbalancing the majority of the internal pressure of the furnace. The excess furnace pressure is the driving force for liquid flow out of the furnace. The advantages of the Roy type tapper over conventional tapping are the greater utilisation of the furnace for higher production and better control of composition than with intermittent tapping. However the Roy type tapper is an equilibrium system and cannot be controlled from a remote location. Consequently this type of tapper cannot handle a feed of variable composition and changes in flowrate must be made by weir or furnace head adjustments.